The present invention relates to a novel apparatus to determine the ability of plastic material to be shaped by the thermoforming process using small amounts of material, under repeatable and controllable process conditions.
Parts and components, which range from those having very large surface areas to those having nominal thicknesses, are often shaped by the thermoforming process. The thermoforming process consists of (1) clamping a sheet of plastic, (2) heating the clamped sheet to plasticating temperature, (3) forming or shaping heated sheet using a suitable mold and counter mold system, (4) cooling a hot formed part, and (5) trimming the edges. In essence, thermoforming is a process of stretching heated sheet to conform to a desired shape either by pulling a stretched sheet over the female mold by applying a vacuum or by pushing heated sheet by a male mold moving downward as in positive forming or upwards as in negative pressure forming.
The ability of plastic material to be shaped by the thermoforming process depends on the rate of change in the strength or the elastic modulus of material with the change in temperature. Thus, highly crystalline materials such as polyamide, polyesters, and polypropylene having sharp melt transition, i.e. very high strength below melting point and very low strength above melting point, are difficult to thermoform, while amorphous materials such as rigid or plasticized PVC, PMMA, PS, ABS, SAN, SMA, which show gradual decrease in strength, with increase in temperature, can be thermoformed relatively easily.
The strength of a plastic material increases with its molecular weight. Hence, materials with a higher initial molecular weight will retain relatively higher strength upon heating than those materials with lower initial strength. The higher molecular weight materials with lower melt elasticity, or narrow molecular weight distribution, cannot be stretched or drawn to a higher level. The ability of a material to stretch also depends on the degree of entanglement. At equivalent molecular weight, molecules with low level of long chain branches such as LDPE tend to draw more than materials with relatively short branches, such as polypropylene. In addition to crystallinity, molecular weight, molecular weight distribution and the length of side chains, thermoformability will also vary with the orientation, amount and type of fillers, the amount and type of plasticizers, the amount and type of pigments, the amount of trim or recycled content, the thickness and weight variation, the degree of residual stresses along and across the extruded web, and sheet thickness. In addition, thermoforming also depends on the ability of material to uniformly absorb heat energy, sheet temperature, applied force, and the ability to resist degradation or embitterment upon heating.
Thus, for a plastic processor, it is very difficult to predict whether a newly developed material or slightly adjusted composition, new lot of extruded sheet stock made of same material, or sheet stock containing varying amount of recycled trim will thermoform well or not; or the proper process conditions the given material can be thermoformed to desired shape.
The actual field scale testing of thermoforming, even though most informative, is cost prohibitive. In many cases, large amounts of material required to make test sheet stock is not available, or equipment to make wide web required for commercial size machine may not be readily available, or enough sheet stock may not be available to adjust process parameters. In other situations, the processor may want to quickly establish starting process conditions for a given material without creating too much of start-up scrape, needs to sort out good lot from a bad lot, or make quick adjustments from virgin to recycled trim blend ratio.
One of the methods used to estimate thermoformability is hot tensile test, described in ASTM D 638, according to which injection molded or die-cut dog-bone shaped sample is stretched at uniform speed at forming temperature. Hot tensile tests are difficult to carry out with any degree of reliability or confidence in data. At forming temperature, uni-axial stretching is not confined to the neck-down portion of the sample. Grip-slip or extrusion of plastic from grips is common. Further, long conditioning time required to achieve desired temperature can induce annealing and stress relaxation, both affecting the measured tensile modulus.
Hot creep is another uni-axial test in which fixtured sample is placed in a heated oven without load, and after it reaches to equilibrium temperature, a very high load is applied instantaneously. High-speed video camera is used to determine time dependent elongation to break at that temperature. Although test is relatively simple, interpretation of the results is difficult. Even though hot creep test data suffers from the same vagarities of the hot-tensile test, hot creep test is more sensitive and provides cleared stress-strain data at high strain rate levels.
The stretchability of material in melt phase is also evaluated using a melt-tensiometer. In this test, a thin strand of material is extruded using a strand die and strand is stretched at uniform and controllable velocity using a pulling device equipped with force sensors. The amount of force required at various velocities is measured and draw velocity at which strand breaks is noted as maximum draw velocity. Even though this test is very useful in comparing melt strength of different materials, it does not reflect the real thermoforming process, which is carried out in semi-solid phase, and in which the material is stretched in all three directions.
Other stretching tests involve inflating a heated circular disk of test material at constant pressure and determining rate of biaxial stretching using high-speed video camera. The result is then used to determine appropriate constants in stress-strain equations. Such test can be used for relatively thin films only and test results are not directly applicable to thermoforming process.
Stress-strain behavior as function of time can be tested using a dynamic mechanical analyzer or DMA. DMA measures relative elastic modulus of material as function of temperature at fixed frequency of applying load or at fixed temperature as function of frequency. Even though DMA requires very small amount of material, and test results are highly accurate and repeatable, DMA is expensive and requires highly skilled personnel to operate and interpret data. Further, it does not reflect actual thermoforming process.
One of the most widely used tests is the sag resistance test. In this test, a rectangular or circular sample of sheet feed stock is clamped between two plates and placed in a heated oven. The time for sample to sag under its own weight by fixed distance or distance sagged for fixed time at given temperature is then determined. Even though easy and least expensive, results of sag tests are specific to geometry, i.e. size and shape of sample, and the size of sample changes during test. Further, most extruded sheet has residual stresses, which tends to relax upon heating it. This negative sag is not accounted for in a typical sag test.
Thus, among a variety of tests available, some are highly precise and repeatable, others are simpler but less precise and lack repeatability, and most of all none truly replicate the actual tri-axial stretching phenomenon taking place in thermoforming process.
The patent literature is replete with thermoforming-related inventions. U.S. Pat. No. 4,034,602 discloses an instrument for determining the complex mechanical response of samples incorporates two parallel sample arms each pivotally mounted at their central portion by flexure pivots of precisely known spring constants. The sample is mounted on one end of each. An electro-mechanical driver acts on the other end of one arm to maintain the arms and sample in mechanical oscillation about the pivots. A displacement transducer senses the mechanical motion. A feedback amplifier between the displacement transducer and the driver maintains the oscillation at a constant amplitude and at a resonant frequency determined primarily by the sample. With this arrangement the driver and displacement sensor are removed from the sample and its usual thermal chamber. This improves the stability of the instrument. At the same time the arms are dynamically balanced about the pivots and hence are relatively insensitive to vibrational upset.
U.S. Pat. No. 5,795,535 discloses a precut die apparatus arranged and adapted for use in a thermoform-trimming method and system to produce plastic molded articles, and to the method and system in which the apparatus is employed. The precut apparatus is positioned, in the method and system, between the form press and the trim station. The precut apparatus is arranged to precut, in a desired and selected manner, the thermoformed sheet material containing thermoformed articles therein, about the periphery of the thermoformed articles. The precut provides for a bridged and joining area of the sheet material to allow for slight movement and adjusting of the molded articles for precise alignment in the punch and die trim step of the thermoforming and trimming operation.
U.S. Pat. No. 4,692,111 discloses an apparatus for forming plastic articles by a thermoform process. A plastic sheet to be formed is die cut to a predetermined shape, heated, and draped over a male mold extending vertically upward. A vacuum is drawn internally of the male mold forming an imprint in the sheet from a die carried on the outer uppermost surface of the mold. A mating female mold is then lowered about the male mold to press the sheet about the conforms of the outer surface of the male mold until the plastic sets, thereby forming the desired object.
U.S. Pat. No. 4,674,972 discloses an apparatus for forming plastic articles by a thermoform process. A plastic sheet is supported between an upper female and lower male mold. The heater assembly is moved horizontally over the sheet, which is heated and caused to conform to the lower molds shape when the upper mold is lowered thereabouts, thus forming the desired object. Vacuum means retain the formed object within the upper mold assembly during separation of the molds and deposits the formed object on the top of the heater assembly upon its repositioning over the lower mold to heat a subsequent sheet. The formed object is carried away from the molds on the top of the heater assembly upon subsequent horizontal retraction of the heater assembly from between the molds. An air jet from an orifice on the top surface of the heater assembly then propels the formed object onto a conveyor belt.
U.S. Pat. No. 4,297,884 discloses a method of and apparatus for the measurement of at least one mechanical property of an elastic material. Young's modulus and/or the internal damping factor of an elastic material are obtained by subjecting an area of the material to a sustained vibration. The presence of the material being tested changes the resonance of a mechanical resonator and determination of the changed resonance peak enables the required elastic characteristics to be obtained. The sample may be subjected to varying tension during testing and can conveniently be vibrated by signals obtained from a variable frequency generator although, preferably, an electronic circuit for vibrating the resonator has a feedback loop to operate as an electro-mechanical auto-oscillator.
U.S. Pat. No. 3,751,977 discloses an analyzing structure for determining properties such as elastic shear modulus and mechanical hysteresis of a material. A pair of mutually spaced holders hold the sample in such a way that the holders are interconnected by the sample, and these holders are in turn carried by driver and driven supports. A drive sets the driver support into vibratory motion so that the latter is transmitted through the sample to the drive support. By detecting the manner in which the driver and driven supports vibrate it is possible to determine properties of the sample. The sample is tested by cyclically generating substantially pure shear forces in the sample, with the power required to sustain the vibrations of the sample at a constant level being measured to determine the damping of the sample and frequency of vibration being measured to determine the modulus of the sample.
While there are a number of patents directed to thermoforming technology, none are related to or disclose a technology for the testing of thermoplastic materials and for determining the propensity and ability of a material to be thermo-formed.